1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image using laser light.
2. Description of the Related Art
Heretofore, laser scanning means with a galvanometer mirror has been widely used in image forming apparatuses, such as a copying machine, a printer and a facsimile machine. In such an image forming apparatus, the galvanometer mirror is designed to be oscillated (swingingly moved or reciprocatingly rotated) within a given angular range in a constant cycle so as to reciprocatingly move a beam of light in a scanning manner to form an image at a high speed.
The galvanometer mirror has the problem that its operating frequency fluctuates depending on an operating condition, such as drive voltage. As measures for solving this problem, Japanese Patent Laid-Open Publication No. 07-154544 discloses a correction technique of comparing a scanning speed of a light beam with a predetermined scanning speed to adjust a frequency of an image-data write clock based on the comparison result. Japanese Patent Laid-Open Publication No. 2001-228434 discloses another correction technique of monitoring an oscillation range of a light beam to adjust a drive current value for a galvanometer mirror in such a manner as to allow the oscillation range to become approximately equal to a predetermined target oscillation range.
The former conventional technique is required to perform a complicate control including the steps of detecting a scanning speed of a light beam, comparing the detected scanning speed with a predetermined scanning speed and adjusting a frequency of an image-data write clock based on the comparison result. The latter conventional technique has a problem about structural complexity due to a sensor necessary for monitoring an oscillation range of the galvanometer mirror.